1. Field of the Invention
The present invention relates to an electrically scanning microwave radiometer mounted in a flying body such as a polar orbit platform and capable if measuring accurately the brightness temperature of an object located in an electrically scanned area.
2. Prior Art
Electrically scanning microwave radiometers are known, as described in "MANUAL OF REMOTE SENSING", second Edition, 612-622, published by American Society of Photogrammetry in 1983.
FIG. 1 schematically shows a block diagram of an example of a conventional electrically scanning microwave radiometer (ESMR). The ESMR has a planar array antenna 100 comprising a number m of slotted waveguides. One end of each of the slotted waveguides is connected to a phase shifter in a phase shifting unit 200. Each of the phase shifters is under the control of a controller 300 and is connected to a low-noise receiver 400 through an appropriate transmission line. The output receiver 400 is supplied to an integrator 500.
A microwave noise emitted by a measured object is received by planar array antenna 100. The antenna temperature T.sub.A received by planar array antenna 100 is expressed by the following equation: ##EQU1## where G(.OMEGA.) is a grain function of planar array antenna 100; T.sub.B (.OMEGA.) is a brightness temperature of the measured object; and .OMEGA. is a solid angle. The direction of beams formed by planar array antenna 100 can be changed to allow a raster scan to be performed by changing the amount of phase shift set in the respective phase shifters from time to time by means of controller 300.
The signal received by planar array antenna 100 is amplified and detected by low-noise receiver 400 and then integrated by integrator 500, resulting in a final output signal. This output signal represents the average brightness temperature of the measured object in the area defined by the antenna beam width.
The temperature resolution .DELTA.T representing the minimum receiving sensitivity of the ESMR is expressed as follows: ##EQU2## where K is a constant determined by the construction of low-noise receiver 400; T.sub.A the antenna temperature expressed by equation (1); T.sub.R the receiver noise temperature of low-noise receiver 400; B the band width of low-noise receiver 400; and .tau. the integration time of integrator 500. As is clear from equation (2), the temperature resolution .DELTA.T of the ESMR may become smaller when the integration time of integrator 500 increases. On the other hand, the brightness temperature of the measured object changes, as shown in FIG. 2, in accordance with the incident angle formed between the beam axis of the receiving antenna and the normal axis of the measured object. Accordingly, even if the same object is measured, when the incident angle varies as the antenna beam is scanned, it may be difficult to distinguish a case where the antenna temperature is changed by the change in the brightness temperature of the measured object from a case where the antenna temperature is changed by a variation in the incident angle. To avoid such a problem, the amount of variation in the incident angle is kept within a predetermined range in a conventional ESMR by limiting the scanned width of planar array antenna 100.
As described above, a receiving planar array antenna of a conventional ESMR performs the raster scan on a measured object. If a wide area is scanned, it seams as if the brightness temperature changes as the incident angle varies during scanning.
The conventional electrically scanning microwave radiometers have some other problems too. One problem is how to use a single polarized wave in conjunction with the use of multiple frequencies in order to measure the same object with differently polarized waves or different frequencies. In particular, while scanning enabling multiple frequencies to be used in an electrically scanning microwave radiometer is one of the main problems remaining to be solved, there has to date been no example of making a practicable multiplei-frequency ESMR. In addition, if the orbit of a flying body such as a polar orbit platform is defined, the velocity of the flying body is determined in accordance with the orbit height. Accordingly, a wider scanning area needs a higher scanning velocity, which makes the integration time of integrator 500 shorter and the temperature resolution poorer.